In medical imaging so-called “hybrid modalities” are becoming increasingly important, for example PET-CT, SPECT-CT, PET-MR and SPECT-MR. The meanings of these abbreviations are as follows:    PET: Positron Emission Tomography    CT: Computer Tomography    SPECT: Single Photon Emission Computed Tomography    MR: Magnetic Resonance tomography
The advantage of these combinations is the connection of a modality with a high local resolution (especially MR or CT) to a modality with high sensitivity (especially SPECT or PET). Reference is made below to a combined PET-MR system. The embodiments of present invention can however be transferred in general to all forms of hybrid modalities or to related measurement processes.
PET uses the particular characteristics of the positron emitter and positron annihilation for quantitative determination of the function of organs or cell areas. In such cases corresponding radio pharmaceuticals which are marked with radio nuclides are administered to the patient before the examination. As they decay, the radio nuclides transmit positrons which after a short distance interact with an electron, which causes so-called annihilation to occur. This produces two gamma quants which fly off in opposite directions (offset by 180°) to each other. The Gamma quants are detected by two opposing PET detector modules within a specific time window (coincidence measurement), which determines the location of the annihilation to a position on the connecting line between these two detector modules.
For verification, the detector module for the PET must generally cover a majority of the gantry arc length. It is subdivided into detector elements of a few millimeters side length. Each detector element, on detection of a Gamma quant, generates an event recording which specifies the time as well as the verification point, i.e. the corresponding detector element. This information is transmitted to high-speed logic and compared. If two events fall within a maximum time spacing, a Gamma decay process on the connecting line between the two associated detector elements is assumed. The PET image is reconstructed with a tomography algorithm, i.e. what is referred to as back projection.
Since MR systems operate with high magnetic fields, the use of compatible materials within these systems is thus required. Particular attention should be paid in the construction of the PET detectors in combined PET systems to the detectors being insensitive to magnetic fields.
US 2007/0102641 A1 describes a combined PET-MR in which lutetium oxyorthosilicate (LSO) is used as a scintillation crystal for converting the Gamma-quants into light and avalanche photo diodes (APD) are used for detection of the light. The APDs are connected to preamplifiers. A ring of such PET detectors is arranged within an MR device. This allows MR and PET data sets to be recorded simultaneously. A comparable arrangement is known from U.S. Pat. No. 7,218,112 B2.
With the frequently used semiconductor amplifiers and semiconductor detectors (Avalanche Photo diodes, APD) the amplification particularly depends on the temperature. Since the components are subjected to temperature variations during operation cooling is required. The supply of cooled air allows the temperature of the amplifiers and photo diodes to be regulated. When air at a constant temperature is used the temperature of the amplifiers is produced from the equilibrium of the generated heat and the heat emitted by the air over the surface of the amplifiers. The cooling can be implemented in the same way for other parts of the detection system.
The APDs are however not just subjected to temperature fluctuations because of their operation. In particular the proximity to the gradient coil and the excitation coil of the MR system caused by the compact design represents an external heat source acting on the APD. The temperature of a gradient coil during operation is typically between 20 and 80° C. These temperature differences also act on the APD and thereby on their amplification. The effects of this heat source can also only be overcome with difficulty using air cooling.